The invention relates to a method of converting a stream of data bits of a binary source signal into a stream of channel bits of a binary channel signal in a transmission system, more specifically, a system for recording and reproducing a recording signal on a record carrier, the bit stream of the source signal being divided into a contiguous sequence of five authorized source words in accordance with the following Table, which authorized source words are converted into the associated channel words of the Table:
______________________________________ Source words Channel words ______________________________________ 10 0100 11 1000 0(1) 00 00(1) 0000 000 100100. ______________________________________
The channel signal obtained in the manner described above is further subjected to what is commonly referred to as a NRZ-M modulation, that is to say, a binary signal is generated which evidences edges corresponding to the "1" bits of the channel signal.
Such a method is disclosed in Electronics Letters, Apr. 28th, 1983, Vol. 19 No. 9, page 323. In this encoding technique, the bit stream of the source signal is divided into a contiguous sequence of a limited number (5) of authorized source words of different lengths, and these different source words are converted into the associated channel words. This encoding technique excells as regards a number of properties which are particularly important for the recording on a record carrier, such as:
1. The minimum pitch between consecutive transitions in the channel signal is limited, so that the required frequency bandwidth is limited.
2. The maximum pitch between consecutive transitions in the channel signal is limited, so that the code is self-clocking, that is to say, that the clock signal required for decoding this channel code can be derived from the bit stream itself.
3. The decoding time window, that is to say, the duration of the time period within which the presence or absence of a signal transition in the channel code must be detected, is very acceptable.
4. The error propagation, that is to say, the number of incorrectly decoded data bits due to a single faulty channel bit, is very small.
5. The electronics required for coding and decoding is not complicated, which is very important, particularly for consumer uses.
As holds for any type of coding, also for the above-described code, the use of a synchronizing word is indispensable for dividing the channel bit stream into unambiguously identifiable information blocks. This synchronizing word must have a channel bit pattern which can unambiguously be identified in the stream of channel bits. On the other hand, it is of course advantageous for this synchronizing word to occupy as little space as possible of the available information space and moreover to require the least possible additional electronics, both in the encoding and in the decoding circuits.
A complication as regards the insertion of a synchronizing word in the above-mentioned code is the fact that because of the use of permissible source words of different lengths during encoding of the data bits preceding the beginning of a synchronizing word, use must be made of data bits which coincide with the first four channel bits of the synchronizing word, as will be explained in greater detail hereinafter. Also during decoding of the channel bits, the first four channel bits of the synchronizing word play a part in the decoding of the last two channel bits of the bit stream preceding the beginning of the synchronizing word. This imposes additional requirements on the pattern of this synchronizing word.